Carrier current telephone system



July 3, 1951 l. MoLNAR CARRIER CURRENT TELEPHONE SYSTEM 7 Sheets-Sheet 1 Filed F'eb. 14, 1947 I mm2 W @man f., A .O m

EO Sgm CMQ@ July 3, 1951 LMOLNAR CARRIER CURRENT TELEPHONE SYSTEM 7 Sheets-Sheet 2 Filed Feb. 14, 1947 Inventor By Imre Molnor 6a/nb, W, MM Ahorneys N .OE

July 3, 1951 MoLNAR CARRIER CURRENT TELEPHONE SYSTEM 7 sheds-sheet 5 Filed Feb. 14, 1947 w @E fm m v m d M. @E w E .m ,m @E N E Q my am@ m, ON M @Zo M I. d ||IJ|I m8 Sm Leomccoo BQ L mm T u @wm :MNE n ses mmm@ 255 l. @5585 f- @ci @v8 IM NB ZE 0mm mw o Al @vm -Il lv .n wzzulv m .O L m8 mmw @mm L T @mm o 6 8 AI 4 v 532mm .Y Y Nmmo www vm wl.. @509:0 v K ,mu i 4| SEE 28am lv TIV 8 o Il A, mm r:E 8m w O m A '7 Sheets-Sheet 4 l. MOLNAR CARRIER CURRENT TELEPHONE SYSTEM July 3, 1951 Filed Feb. 14, 1947' AIIornevs l. MOLNAR 2,559,165

v sheets-sheet 5 July 3, 1951 CARRIER CURRENT TELEPHONE: SYSTEM Filed Feb. 14, 1947 Invemor Imre Molnor Attorneys July 3, 1951 l. MOLNAR CARRIER CURRENT TELEPHONE SYSTEM '7 Sheets-Sheet 6 Filed Feb. 14, 1947 July 3, 1951 l. MoLNAR CARRIER CURRENT TELEPHONE SYSTEM '7 Sheets-Sheet 7 Filed Feb. 14, 1947 Inventor Imre Mo`|nur Patented `uly 3, 17951 f CARRIER CURRENT TELEPHONE SYSTEM Imre Molnar, Chicago, Ill., assgnor to Automatic Electric Laboratories, Inc., Chicago, Ill., a corporation of Delaware Application February 14, 1947, Serial No. 728,422

32 Claims.

The present invention relates to carrier current telephone systems and more particularly to such systems incorporating carrier current units permitting improved duplex control and supervision.

It is an object of the invention to provide an improved carrier current telephone system incorporating carrier current units, each unit being provided with separate transmitting and receiving signal channels and separate transmitting and receiving communication channels, whereby the transmission and reception of straight carrier signals for signaling purposes in no way interfere with the transmission and reception of modulated carrier signals for communication purposes.

Another object of the invention is to provide in a telephone system improved trunk circuits utilizing carrier current equipment and permitting the setting up and subsequent release of telephone connections in a straightforward manner.

A further object of the invention is to provide in a telephone system comprising two exchanges, each including an operator switchboard, an improved arrangement utilizing carrier current equipment for setting up connections between the operator switchboards.

A further object of the invention is to provide in a telephone system comprising a first exchange including an operator switchboard and a second exchange including automatic switching apparatus and a plurality of substations, an improved arrangement utilizing carrier current equipment for setting up connections from the operator switchboard by way of the switching apparatus to the subscriber substations and for setting up connections from the subscriber substations by way of the switching apparatus to the operator switchboard.

A still further object of the invention is to provide in a telepho-ne system comprising two exchanges, each including automatic switching apparatus and a plurality of subscriber substations, an improved arrangement utilizing carrier current equipment `for setting up connections from calling subscriber substations in either of the exchanges by way of the switching apparatus in the two exchanges to call subscriber substations in the other of the exchanges.

Further features of the invention pertain to the particular arrangement of the circuit elementsl of the system, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, Will best be understood by reference to the following specification taken in connection with the accompanying drawings in which Figure 7 is a diagrammatic illustration of a telephone system embodying the present invention and comprising a manual exchange E0 and an automatic exchange 2U; Fig. 8 is a diagrammatic illustration of a modied telephone system embodying the present invention and comprising a manual exchange 30 and a manual exchange 40; Fig, 9 is a diagrammatic illustration of a further modied telephone system embodying the present invention and comprising an automatic exchange 50 and an automatic exchange Figs. 1 to 5, inclusive, taken together illustrate the details of the apparatus incorporated in the telephone system illustrated in Fig. 7 and embodying the features of the invention as briefly outlined above; and Fig. 6 illustrates the mode of combining Figs. 1 to 5, inclusive, to form a unified diagram.

More particularly, Fig. 1 illustrates the details of one of the cord circuits disposed at the operator switchboard and the details of one of the trunk circuits, both disposed in the manual exchange l0 in the telephone system of Fig. 7; Fig. 2 illustrates the details of one of the trunk circuits disposed in the automatic exchange 2|] in the telephone system illustrated in Fig. 7; Fig. 3 illustrates the general arrangement of the automatic switching apparatus arranged in the automatic exchange 20 in the telephone system of Fig. '7; and Figs. 4 and 5 respectively illustrate diagrammatically the carrier equipment disposed in the manual exchange I 0 and in the automatic exchange 20 and interconnected by a line extending therebetween in the telephone system illustrated in Fig. 7

Referring now more particularly to Fig. 7, the telephone system there illustrated comprises a manual exchange l serving a group of subscriber substations, not shown, and an automatic exchange 2E) serving a plurality of groups of sub-l scriber substations, one of the groups including the subscriber substation T. The manual exchange l0 and the automatic exchange 20 are interconnected by a line 400 extending therebetween. The west end of the line 400 terminates in Voice frequency equipment 4|2W disposed in or beyond the manual exchange I0; and the east end of the line 400 terminates in voice frequency 'equipment 5l2E disposed in or beyond the automatic exchange 20. The manual exchange l0 also comprises an operator switchboard |00, while the automatic exchange 2U comprises automatic switching apparatus 300.

The manual exchange l and the automatic exchange 20 are interconnected via the line 400 by a three channel carrier system comprising common channel carrier equipment and individual channel carrier equipment disposed in each of the exchanges mentioned. Moreparticularly, the common channel carrier equipment disposed in the manual exchange I0 comprisesa high pass line filter 4I3 directly connected to the west portion of the lineMlU, -areceivingdirection filter M4 and a transmitting direction lter 4I5 both connected tothe high pass-line iilter 4 I 3, a receiving line amplier 4 I 6 connected to the receiving direction filter 9M, and a transmitting line amplifier 4 I 1 connectedto the transmitting direction iilter 4|5. Also, a low pass line lter dll is connected between the line 400 and the voice frequency equipment 412W. Similarly, the common channel carrier equipmentdis'posed in the automatic exchange 20 comprises a high pass line lter'5I3 directly connected to the east portion of the line 400, a receiving direction lter 5I4 and a transmitting direction filter v5I5 both connected to the high pass line filter 5I3, a receiving line amplifier 5|6 connected to the receiving direction filter 5|4, and a transmitting line amplifier 5I1 connected to the transmitting direction filter 5|5. Also, a low pass line filter 5II is connected between the line 400 and the voice frequency equipment SIZE.

VThe individual channel carrier equipment disposed in the manual exchange I0 comprises three individual units of carrier equipment 40 IW, 402W and 403W commonly connected to the receiving Vline amplifier-M6 and to the transmitting line amplifier M1, and three respectively associated trunk circuits IUITC, IllZTC` and I03TC interposed between the operator switchboard |00 and the individual channel carrier equipment mentioned. Similarly, the individual channel carrier equipment disposed in the automatic exchange comprises three individual units of carrier equipwith the outgoing trunk 3|!) and the incoming trunk 320 individual thereto. Y

Itrwill be understood that in the carrier system the individual channel carrier equipment 40|W, 402W and 403W in the manual exchange IIJ are respectively pairedwith the individual channel carrier equipment 50IE, 5021i. and 503E disposed in the automatic exchange 20; which three pairs of individual channel carrier equipment comprises the respective rst, second and third channels mentioned.

Preferably, the carrier system comprises a type 32 Lenkurt unit, wherein the individual channel carrier equipment 503151 transmits a carrier frequency of 9.35 kc. for signaling purposes, transmits the lower side band of the carrier frequency 9.35 kc. (6.5 to 9.1 kc.) for communication purposes, receives a carrier frequency of 28.45 kc. for signaling purposes, and receives the lower side band of the carrier frequency 28.45 kc.-

4 (25.6 to 28.2 kc.) for communication purposesj the individual channel carrier equipment 502E transmits a carrier frequency of 12.55 kc. for signaling purposes, transmits the lower side band of the carrier frequency 12.55 kc. (9.7 to 12.3 kc.) for communication purposes, receives a carrier frequency of 20.85 kc. for signaling purposes, and receives the lower side band of the carrier frequency 20.85 kc. (18.0 to 20.6 kc.) for communication purposes; the individual channel carrier equipment 50|E transmits a carrier frequency of 15.95 kc. for signaling purposes, trans- Vmits'theilower side band of the carrier frequency 15.95 kc. (13.1 Vto 15.7 kc.) for communication purposes,receives -a carrier frequency of 24.55

kc. for-signalingpurposes, and receives the lower side bandof the carrier frequency 24.55 kc. (2117 to 24.3'kc.) for communication purposes; the individual channel carrier equipment IiBZW transmits a carrier frequency of 20.85 kc. for signaling purposes, transmits the lower side band of the-carrier frequency'20185fkc. (18.0'to `.20.6 kc.) for communication purposes, receives a carrier lfrequency of 12.55 kc. for signaling purposes, and receives the lower side band of the carrier frequency 12.55 kc. (9.7 to 12.3 kc.) for communication purposes; the individual channel carrier equipment 40IW transmits vacarrier frequency of 24.55 kc. for signaling purposes, transmits the lower side band ofthe carrier frequency 24.55110.

y(21.7 to 24.3 kc.) for communication purposes,

receives a carrier frequency of 15.95 kc. for signaling purposes, and receives Vthe lower sideband of the carrier frequency 15.95 kc. (13.1 to 15.7 kc.) for communication-purposes; -and rthe individual channel carrier equipment 403W transmits a carrier frequency of 28.45 kc. for signaling purposes, transmits the lower side band of the carrier frequency-28-45 kc. (25.6 to 28.2 kc.) for communication purposes, receives a carrier frequency of V9.35 kc. for signaling purposes, and receives the lower side band of the carrier frequency 9.35 kc. (6.5 to v9.1 kc.) for communication purposes. Y

As shown in Fig. l, the operator switchboard |00 comprises a number-of cord circuits, including the cord-circuit 10|, and standard common equipment, including a headset IIS, a dial DI08 and an interrupter generator |01. Further, the cord circuit |01 vcomprises two plugs P|02 and PI03, a dial key KIM operative to connect the dial D|0 8 thereto, a'talkkey YK|Il5 operative to connect the headset ||3 thereto, and a ring key KIIlB Ioperative to connect the interrupter generator |01 thereto. Also, the cord circuit ISI comprises a supervisory lamp LIII associated .with-the plug P|02, a Ysupervisory lamp LI|2 associated with the plug PI03, a supervisory relay RIBI] associated with the supervisory-lamp LIII, a supervisory relay RMU associated with the supervisory lamp'LI I2, anda dial relay RI20 associated with the dial lieyKIEl and with the ldial DI08.

The trunk circuits IilITC, ISZTC and |03TC Aare identical; thetrunk circuit |03TC shown in Fig. 1 being connected-to the carrier equipment 453W shown in Fig-4, as previously noted. The

.trunk circuit ISTC terminates in a jack JI5I at the operator switchboard |00 and comprises a line lamp LI52, a busy lamp L'I53 and a relay group, including a sleeve relay vRII, a jack relay Rl and'a signal-in relay -RI8I).

The trunk circuits' ZIJITC, 202TCand 203'IC areidentical; the trunk circuit 203TC shown in Fig. 2 being connected to the carrier equipment 50313 shown in Fig. 5, as previously noted. lThe trunk circuit 2il3TC term-inates in an outgoing trunk 3io accessible to the automatic switching, apparatus 390 and an incomingl trunk 329 having access to the automatic switch apparatus 340, as previously noted; and comprises a relay group, including a hold relay R2 0, a hold relay R223, a timer relay R238, a dial relay R240, a timer relay R, a tone relay R250, an answer relay R265, a switch relay RZ'FG, a signal-in relay R280, a supervisory relay R285., a line relay R230 and a reverse battery relay R295.

Asshown in Fig. 3, the subscriber substation T has a subscriber line 330 extending thereto, with which a line circuit 335I is individually associated. The automatic switching apparatus 39|)` comprises a plurality of groups of nderselector links, each provided with an associated distributor, and corresponding to the groups of subscriber substations. One, of the groups of iinder-selector links includes the finder 342 and the selector 34| and is provided with the individually associated distributor 34B.; which group of finder-selector links is associated withV the group of subscriber substations, including the subscriber substation T. Also, the automatic switching apparatus comprises a plurality of groups of connectors respectively associated with the groups of subscriber substations. One of the groups of connectors includes the connector 3l@ and is associated with the group of subscriber substations, including the subscriber substation T.- Also, the automatic switching apparatus 359 comprises a group of outgoing selectors, including the outgoing selector 354, and three incoming selectors, including. the incoming selector 390. More particularly, the iinder 342 of the finderselector link illustrated has access tothe subscriber line 333 extending to the subscriber substation T; while the selector 34| of the iinderselector linlr illustrated has access to the connector 310 by way of the associated trunk 3ll, andv has access to the outgoing selector 359k by way of the associated trunk Also, the outgoing selector 353 has access to the outgoing trunk 31|] extending into the trunk circuit 243TC while the incoming trunk 329 extending from the trunk circuit 203TC is terminated in the incoming selector 360. Finally, the incoming selector- 364v has access-to the connector 31D by way of the associated trunk 31 I.

Preferably, the nders 342, etc., the selectors 34|., etc., the outgoing selectors 355, etc., the incoming selectors 363, etc. and the connectors 370, etc. are of the well-known Strowger types. Also, the finder 342 comprises a plurality of sets of post normal switch springs, one of which is indicated at S344, which sets of switch springs are actuated selectively in response to operation of the linder 342 different numbers of steps in the vertical direction in the selection of the corresponding ten line groups; and which sets of switch springs'are connected to individually associated class tone conductors. Specifically, the set of switch springs S344 is actuated into closed circuit'relation in order to connect the associated class tone conductor C346 tothe associated condenser 345 when the finder 342 is operated apredetermined number of steps in the vertical direction tcselect the ten line group of subscriber lines, including the subscriber line 334, extending to the subscriber substation T illustrated. Thus itl will vbe understood that the subscriber substation T is in-a given ten line grouprendered a particular class of service, as indicated by the signal present on the class tone conductor C346.

Similarly, the selector 34| comprises a set of post normal switch springs S343 which is operated into closed circuit relation when the selector 34| is operated to a predetermined level to select the group of trunks, including the trunk 35|, and indicative of a trunk call. from the automatic exchangeZ to the manual exchange I0.

Referring to Fig. 4, the low pass filter 4I| is arranged to pass audio frequency signals on the line 403 to the voice frequency equipment 4|2W but to block all of the high frequency carrier signals in order to prevent interference with the normal operation of the voice frequency equipment 4 |2W. The high pass line filter 4|3 is arranged to pass high frequency carrier signals in the overall range 6.5 to 28.45 kc. and to block the passage of audio frequency signals in order to prevent interference with the carrier system.

The receiving direction iilter 4|4 is arranged topass carrier frequency signals toward the lower end of the overall carrier frequency range and particularly the carrier frequencies between 6.5y

and 15.95 kc., and toiblock carrier frequency signals toward the upper end of the overall carrier frequency range and particularly the carrier frequency between 18.0 and 28.45 kc. On the other hand, the transmitting direction filter 4|5 is arranged to pass carrier frequency signals towardthe upper end of the overall carrier frequency range and particularly the carrier frequencies between 18.0 and 28.45/ kc., and to block carrier frequency signals toward the lower end of the overall carrier frequency range and'particularly the carrier frequencies between 6.5 and 15.95 kc.

The carrier equipment' 403W comprises, as shown in Fig. 4, a receiving communication channel, including a receiving band pass filter 42| connected to the receiving line ampliiier 4|3and arranged to pass the lower side band of the carrier frequency 9.35 kc. (6.5 to 9.1 kc.), an attenuation equalizer 422 connected to the receiving band pass filter 42|, a gain control 423, connected to the attenuation equalizer 422, and a demodulator 424 connected to the gain control 423. Further, a carrier frequency oscillator 428- having an output frequency of 9.35 kc. is connected by an oscillator pad 427 to the demodulator 424; and the demodulator 424 is connected to a receiving low pass iilter 425 arranged to pass audio frequency signals; Which, in turn, .is

connected by a receiving ampliiier 426 to a hybrid coil 43D provided with a balancingnetwork 43|. Further, the hybrid coil 43|! is connected by way of the conductors CESI and C|92 to the trunk circuit |93TC, as previously noted.

Also, the carrier frequency equipment 483W comprises a receiving signal channel, includingv a signal receiving iilter 45| connected to the re.-

ceiving line amplifier 4|6 and arranged to pass` the carrier frequency 9.35 kc.; which, in turn,

is connected by way of a receiving amplier 452y conductor C|34 extending to the trunk circuit.r |03TC. However, when thereceiving signal channel receives the'carrier frequency of 9.35'kc., thev rectifier 453 is controlled by the receiving amplifier 452 to produce a current that is supplied to the signal bias control 454, whereby the bias applied to the D. C. amplifier 455 is increased in order to cause the output current thereof to be reduced sufficiently to bring about the restoration of the signal-in relay R455, Upon restoring, the signal-in relay R455` completes, at the contacts 451, the previously mentioned path for applying ground potential tothe signal-in conductor Y CI94 extending to the trunk circuit |63TC. Ac-

cordingly, as signal pulses of 9.35 kc. carrier frequency are received by the receiving channel, the signal-in relay R455 follows, repeating, at the contacts 451, corresponding ground impulses over the signal-in conductor C|94 extending to the trunk circuit |53TC.

Further, the carrier equipment 403W comprises a transmitting communication channel including a transmission low pass filter 44| connected to the hybrid coil43 and arranged to pass audio frequencies; which, in turn, is connected by way of an input pad 442 to a modulator 443. A carrier frequency oscillator 44'8 having an Youtput frequency of 28.45 kc. is connected by an oscillator pad 441 to the modulator 443; and the modulator 443 is connected by an output pad 444 to an attenuation equalizer 445. The attenuation equalizer 445 is connected to a transmission band pass filter 446, arranged to pass the lower side band of the carrier frequency 28.45 kc. (25.6' to 28.2 kc.) and to block the upper side band of the carrier frequency 28.45 kc.; which, in turn, is connected to the transmission line amplifier 4|1.

Finally, the carrier equipment 453W comprises a transmission signal channel including a normally restored signal-out relay R46| connected to the signal-out conductor C193 extending to the trunk circuit llliTC; which signal-out relay R46| is adapted to be operated in order to close, at its contacts 452, the plate circuit for a transmission amplifier 453; which, in turn, is connected to the carrier frequency oscillator 448 by way of the oscillator pad 441. Also, the transmission amplifier 453 is connected by way of a signal transmission pad 454 to a signal transmission filter 455 arranged to transmit the carrier frequency 28.45 kc. Finally, the signal transmission lter 465 is connected to the transmission line amplifier 4|1. Accordingly, as the signal-out relay Riill is operated and then restored it completes and then interrupts, at `the contacts 452, the plate circuit extending to the transmission amplifier 453, whereby corresponding pulses of 28.45 kc. carrier are transmitted from the transmission amplifier 463 via the signal transmission pad 464, and thence through the signal transmission filter 465 into the transmission line amplifier 4|1.

Referring to Fig. 5, the low pass filter 5|| is arranged to pass audio frequency signals on the line 403 t the voice frequency equipment 5l2W but to block all of the high frequency carrier signals in order to prevent interference with the normal operation of the voice frequency equipment |2E. The high pass line lter 5|3 is arranged to pass high frequency carrier signals in theoverall range 6.5 to 28.45 kc. and to block the passage of audio frequency signals in order to prevent interference with the carrier system. The receiving direction filter 5|4 is arranged to pass carrier frequency signals toward the upper end of the overall carrier frequency range and particularly the carrier frequencies between 18.0 and 28.5 kc., and to block carrier frequency signals toward the lower end of the overall carrier frequency rangev and particularly the carrier frequencies between 6.5 and 15.95 kc. On the other hand, the transmitting direction filter 5|5 is arranged to pass carrier frequency signals toward the lower end of the overall carrier frequency range and particularly the carrier frequencies between 6.5 and 15.95 kc., and to block carrier frequency signals toward the upper end of the overall carrier frequency range and particularly theV carrier frequencies between 18.0 and 28.5 kc.

The carrier equipment .5535'. comprises, shown in Fig. 5, a receiving communication channel, including a receiving band pass filter 52| connected to the receiving line amplier 5|5 and arranged to pass the lower side band of the carrier frequency 28.45 kc. (25.6 to 28.2 kc.), an attenuation equalizer 522 connected to the receiving band pass. filter 52|, a gain control 523 connected to the attenuation equalizer 522, and a demodulator 524 connected to the gain control 523. Further, a carrier frequency oscillator 528 having an output frequency of 28.45 kc. is connected by an oscillator pad 521 to the demodulator 524; and the demodulator 524 is connected to a receiving low pass filter 525 arranged to pass audio frequency signals; which, in turn, is connected by a receiving amplifier 526 to a hybrid coil 535 provided with a balancing network 53|. Further, the hybrid coil 530 is connected by way of the conductors C20| and C202 to the trunk circuit 23TC, as previously noted.

Also, the carrier frequency equipment 503E comprises a receiving signal channel, including a signal receiving filter 55| connected to the receiving line amplifier 5|5 and arranged to pass the carrier frequency 28.45 kc.; which, in turn, is connected by way of a receiving amplifier 552 to a rectifier 553. The output of the rectifier 553 is connected by way of a signal bias control 554 to a D. C. amplifier 555, the output of which is connected to a signal-in relay R556. The D. C. amplifier 555 normally supplies sufficient current to the winding of the signal-in relay R555 to maintain the latter relay in its operated position, Vwhereby the signal-in relay R556 maintains interrupted, at its contacts 551, an obvious path for applying ground potential to the signal-in conductor C254 extending to the trunk circuit 203TC. However, when the receiving signal channel receives the carrier frequency of 28.45 kc., the rectifier 553 is controlled by the receiving amplifier 552 to produce a current that is supplied to the signal bias control 554, whereby the bias applied to the D. C. amplifier 555 is increased in order to cause 'the output current thereof to be reduced sufficiently to bring about the restoration of the' signal-in relay R555. Upon restoring, the signalin relay R555 completes, at the contacts 551, the previously mentioned path for applying ground potential to the signal-in conductor C204 extending to the trunk circuit 2U3TC. Accordingly, as signal pulses of 28.45 kc. carrier frequency are received by the receiving channel, the signal-in relay R556 follows, repeating, at the contacts 551. corresponding ground impulses over the signal-in conductor C204 extending to the trunk circuit 203TC.

Further, the carrier equipment 403W comprises a transmitting communication channel including a transmission low pass filter 54| connected to the hybrid coil 530 and arranged to pass audio frequencies; which, in turn, is connected by way ofy an input pad 542 to a modulator 543. A carrier frequency oscillator 548 having an output frequency of 9.35 kc. is connected by an oscillator pad 1541 to the modulator 543; and the modulator 543 is connected by an output pad 544 to an attenuation equalizer 565. The attenuation equalizer 545 is connected to a transmission band pass lter 546, arranged to pass the lower side band of the carrier frequency 9.35 kc. (6.5 to 9.1 kc.) and to block the upper side band of the carrier frequency 9.35 kc.; which, in turn, is connected to the transmission line amplifier 5 I 1.

Finally, the carrier equipment 503E comprises a transmission signal channel including a normally restored signal-out relay R56| connected to the signal-out conductor C2 |33 extending to the trunk circuit 2|]3TC; which signalwout relay R56I is adapted to be operated in order to close, at its contacts 562, the plate circuit for a transmission amplifier 563; which, in turn, is connected to the carrier frequency oscillator 548 by way of the oscillator pad 541. Also, the transmission amplifier 563 is connected by way of a signal transmission pad 564 to a signal transmission lter 565 arranged to transmit the carrier frequency 9.35 kc. Finally, the signal transmission lter 565 is connected to the transmission line amplifier 5 I 1. Accordingly, as the signal-out relay R56I is operated and then restored it completes and then interrupts, at the contacts 562, the plate circuit extending to the transmission amplifier 563, whereby corresponding pulses of 9.35 kc. carrier are transmitted from the transmission amplier 563 via the signal transmission pad 564, and thence through the signal transmission lter 565 into the transmission line amplier 5 1.

A better understanding of the connection and arrangement of the apparatus incorporated in the telephone system illustrated in Fig. 7 and briefly described above will be facilitated from a consideration of the extension of connections involving the manual exchange I and the automatic exchange 26.

Considering now the extension of a connection between the manual exchange I0 and the automatic exchange 20, and assuming that a call has been extended from a calling subscriber substation, not shown, in the manual exchange I6 to the operator switchboard |60 therein and that the operator at the switchboard Ill!) has answered the call utilizing the cord circuit IGI, at this time the plug PI63 of the cord circuit IUI has been inserted into the jack terminating the subscriber line extending to the calling subscriber substation and the talk key K| has been opcrate-d, in order to connect the operator headset I|3 to the cord circuit |0I. At this time the operator at the switchboard |00 learns that the subscriber at the calling subscriber substation in the manual exchange I6 desires to converse with a subscriber in the automatic exchange 26, such, for example, as the subscriber at the called subscriber substation T. The operator at the switchboard |60, observing the condition of the trunk circuits lQITC, |62TC and I63TC, as indicated by the corresponding busy lamps, selects an idle one of the trunk circuits mentioned and inserts the plug PI62 of the cord circuit |6| into the corresponding terminating jack. Assuming that the trunk circuit IGSTC is idle at this time, the operator at the switchboard |00 inserts the plug P962 of` the cord circuit iil into the corresponding jack J I 5I terminating the trunk circuit mentioned; the trunk circuit I03TC being indicated 10 as idle at this time by the unlighted busy lamp LI 53 individual thereto.

When the plug P|||2 is inserted into the jack J I5| the contacts I5I thereof are operated, completing an obvious circuit for operating the jack relay R|16 in the trunk circuit |63TC. Also, a series circuit, including the contacts |2I, is completed for energizing the supervisory relay RISO in the cord circuit IUI and the lower winding of the sleeve relay RISO in the trunk circuit I03TC, whereby the sleeve relay R| operates. However, the supervisory relay RI30 does not 0perate as it is of the marginal type. Upon operating, the jack relay R|1|l completes, at the contacts |13, an obvious circuit for illuminating the busy lamp LI53, thereby to indicate the busy condition of the trunk circuit I03TC; and interrupts, at the contacts |15, an obvious path for short-circuiting the upper winding of the signal-in relay RIBI) in order to increase the speed of response of the latter relay. Also,.the jack relay RI1I) completes, at the contacts |12, a circuit, including the contacts |8I, for energizing the upper Winding of the sleeve relay RIG!) in multiple with the lower winding thereof and in series With the winding of the supervisory relay R|30, wherebythe supervisory relay RI36 operates. Upon operating, the supervisory relay R|3c completes, at the contacts |3I, an obvious circuit for illuminating the supervisory lamp LI I I in the cord circuit IUI. Finally, upon operating, the jack relay RI1|i completes, at the contacts |1I, a circuit, including the contacts |63 and |66, for applying battery potential by way of the resistor |68 to the signal-out conductor CI93, assuming that the dial key KIM in the cord circuit |0| is operated at this time; which application of resistance battery potential to the signalout conductor C|93 completes an obvious circuit for operating the signal-out relay R46| in the carrier equipment 463W. The above-mentioned circuit extends from battery by way of the resistor |68, the contacts |66, the ring of the jack J |5I, the ring of the plug PIIJ2, the impulse contactpIIO of the dial DI, and the upper contacts of the dial key KI04 to the tip of the plug PIUZ, and therefrom by way of the tip of the jack JI5| and the contacts |63 and |1| to the signal-out conductor CI93. Upon operating, the sleeve relay RI60 completes, at the contacts IBI, a multiple circuit, including the contacts I1I, for applying battery potential by way of the resistor |61 to the signal-out conductor CI93 and then interrupts, at the contacts |63 and |66, the above-traced original path for applying battery potential by way of the resistor |66 to the signal-out conductor C| 93. Also, the sleeve relay RI60 interrupts, at the contacts |64, a normally completed termination circuit, including the resistor |69, between the line conductors CI 9| and CI92 extending between the trunk circuit |||3TC and the hybrid coil 436 in the carrier equipment 463W. Finally, the sleeve relay RIB!) completes, at the contacts |62 and |65, a connection between the line conductors C|9I and C |62 and the tip and ring of the jack J I5| and, consequently, to the tip and ring of the plug PIOZ, which at this time are operatively connected together by way of the upper contacts of the dial key Kw4 and the impulse contacts IIU of the dial DHiS.

The operator at the switchboard |60 thenpro-V ceeds to dial the iirst digit of the directory number of the called subscriber substation T in. the automatic exchange 20. When the dial D|08 is 11*v thus'operated the ofi-normal springs 109 thereof are rst actuated into engagement, thereby to complete a circuit, including the lower contacts of the dial key K101i, forV operating the dial relay R120 in the cord circuit101. Upon operating, the dial relay R120 interrupts, at the contacts 121, the previously traced series circuit for energizingV the winding of the supervisory relay R130 in the cord circuit 101 and the upper and lower multiply connected windings of the sleeve relay R160 in the trunk circuit 103TC, whereby the relays mentioned restore. Upon restoring, the supervisory relay R130 interrupts, at the contacts 131, the previously mentioned circuit for illuminating the supervisory lamp L111 in the cord circuit 101;

Y and, upon restoring, the sleeve relay R160 recompletes, at the contacts 163 and 166, the previously tracedV original path for applying battery potential by wayY of the resistor 168 to the signal-out conductor C193, which path includes the upper contacts of the dial key -K104 and the impulse contacts 110 of the dial D108, both in the cord circuit 101, as previously noted. Further, the sleeve relay R160 interrupts, at the contacts 161, the previously traced alternative Vpath for applying battery potential by way of the resistor 111 to the signal-out conductor C193; interrupts, at the contacts 162 and 165, the previously traced path, `including the `upper contacts of the dial key K104 and the impulse contacts 110 of the dial D108 in the cord circuit 101, across the line conductors C191 and C192; and recompletes. at the contacts 164, the previously' mentioned termination circuit including the resistor 169 between the line conductors C191 and C192. At this time-battery potential is applied by way of the resistor 168-to the signal-out conductor C193 over theaboVe-traced path including the impulse contacts 110 of the dial D108 in the cord circuit 101. Accordingly, as the dial D108 is returned to its normal position the impulse contacts 110 thereof are operated one or more times, corresponding to the number of impulses in the digit dialed, eecting interruption and then recompletion a corresponding number oftimes of the above-traced path for applying battery potential by way of the resistor 168 to the signal-out conductor C193,

whereby the signal-out relay R461 in the carrier equipment 403W follows the impulses of the present digit. At the conclusion of the first digit when the dial D108 in the cord circuit 101 is returned to its normal position, the impulse contacts 1 10 thereof are maintained in closed circuit position and the off-normal contacts 109 thereof are actuated intoY disengagement in order to interrupt the circuit for maintaining operated the dial relay R120 in the cord circuit 101. Shortly thereafter the dial relay R120 restores, as it is ofv the slow-to-release type, recompleting, at the contacts 121, the previously traced series circuit for energizing the winding of the supervisory relay R130 in the cord circuit 101 and the multiply connected upper and lower windings of the sleeverelay R160 in the trunkcircuit 103TC, causing the relays mentioned to reoperate. Upon reoperating, the supervisory relay R130 again completes, at the contacts 131, the previously mentioned circuit for illuminating the supervisory lamp VL111; and, upon reoperating, the sleeve relay R160 recompletes, at the contacts 161, the previously traced alternative path for applying battery potential by way of the resistor v161 to the signal-out conductor C193; and interrupts, at the-.contacts 163 and 166, the previously traced original path for applying battery potential byl Way of the resistor 168 to the signal-out conductor C193. In a similar manner the operator at the switchboard 100 dials the second and third digits of the directory number of the called subscriber substation T in the automatic exchange 20 and then, at the conclusion of the dialing of the third digit mentioned, she restores the dial key K104 to its normal position, thereby interrupting the previously traced bridge including the impulse contacts 110 of the dial D108 between the tip and the ring of the plug P162 of the cord circuit 101. At this time the tip and the ring of the plug P102 are operatively connected by way of the condensers 132 and 133 to the tip and the ring of the plug P103 and, consequently, by way of the contacts of the talk key Kl to the operator headset 113.

In view of the foregoing explanation of the manner in which the operator at the switchboard 100 seizes the trunk circuit 103TC utilizing the cord circuit 101, it will be understood that resistance battery lpotential is first steadily applied to the signal-out conductor C193; and then application of resistance battery potential to the signal-out conductor C193 is interrupted, at three different times, one or more times each corresponding to the impulses in the three digits mentioned; and then steady resistance battery potential is reapplied to the signal-out conductor C193. Accordingly, the signal-out relay R461 in the carrier equipment 403W operates steadily; then follows the impulses of the three spacedrapart digits mentioned; and then operates steadily again. Each time the signal-out relay R461 operates and then restores it completes and then interrupts, at the contacts 462, the plate circuit of the transmission ampliiier 463 connected by way of the oscillator pad 441 to the carrier frequency oscillator 448, producing a carrier frequency of 28.45 kc.; whereby the transmission amplifier 463 operates to project and then to interrupt the 28.45 kc. carrier frequency over the signal transmission pad 464 through the signal transmission filter 465 to the transmission line amplier 41'1. The signal transmission pad 464 establishes the input of the transmission line amplifier 41'1 at the required level; and the signal transmission lter 465 vinsures that only the 28.45 kc. carrier frequency is included in the input of the transmission line amplifier 41'1.

Each time the transmission line amplifier 411 receives the 28.45 kc. carrier signal it amplies the input carrier signal and passes a corresponding amplied carried signal through the transmission direction filter 415 and the high pass line filter 413 to the line 400. The transmission direction filter 415 passes the frequencies 18.0 to 20.85 kc. extending toward the upper end of the overall carrier frequency range; while the high pass line lter 413 passes the frequencies 6.5 to 28.45 kc. extending over the entire carrier frequency range. The low pass lter 41 1 blocks the passage of all carrier frequencies on the line 400 to the voice frequency equipment 412W; and the low pass filter 511 blocks the passage of all carrier frequencies on the line 400 to the voice frequency equipment 512E. The 28.45 kc. carrier frequency on the line 400 passes through the high pass line iilter 513 and the receiving direction lter 514 to the receiving line amplier 516. The high pass line iilter 513Y passes the frequencies 6.5 to 28.45 kc. extending over the entire carrier frequency range; while the receiving direction filter 514 passes the frequencies 18.0 to 20.85 kc. extending toward the upper end of the overall car-V rier frequency range. The 28.45 kc. carrier frequency supplied to the receiving line amplifier B is amplified and passes through the signal receiving lter 55| to the receiving amplier 552. The signal receiving lter 55| passes the 28.45 kc. carrier frequency and the amplified signal from the receiving amplifier 552 is rectified by the rectier 553 in order to produce a bias that is passed to the signal bias control 554; which, in turn, governs the D. C. amplier 555 in order to cause the D. C. amplifier 555 to reduce the output current thereof supplied to the signal-in relay R556, whereby the latter relay restores. Each time the 28.45 kc. carrier signal is cut oi to the input of the transmission line amplier M1 the carrier signal is removed from the line 400, permitting the signal bias control 554 to re-establish the original bias provided to the D. C. amplier 555, whereby the output of the D. C. amplifier 555 is increased in order to bring about reoperation of the signal-in relay R555. Accordingly, each time the signahout relay RMI in the carrier equipment 403W operates, the signal-in relay R556` in the carrier equipment 503E restores. Accordingly, the signal-in relay R556 in the carrier equipment 503E restores steadily; then follows the impulses of the three spaced apart digits mentioned; and then restores steadily again.

Each time the signal-in relay R556 restores and then yreoperates it completes and then interrupts, at the contacts 551, an obvious path for applying ground potential to the signal-in conductor C204 extending into the trunk circuit 203'I'C, in order to cause operation and then restoration of the signal-in relay R280 in the trunk circuit mentioned. Accordingly, the signalin relay R280 operates steadily; then follows the impulses of the three spaced apart digits mentioned; and then operates steadily again. Upon rst operatingthe signal-in relay R280 completes, at the contacts 28|, an obvious path for short-crcuiting the resistor 221 and the series connected condenser 225, and completes, at the contacts 283, an obvious circuit for operating the hold relay R223. Upon operating, the hold reu lay R220 completes, at the contacts 224, a circuit, including the contacts 2|5, for energizing the lower winding of the switch relay R210, thereby to cause the latter relay to operate. Upon operating, the switch relay R210 interrupts, at the contacts 21| and 2M, the normal connections between the line conductors C20! and C202 extending from the hybrid coil 530 in the carrier equipment 503E and the line conductors C3| and C3 2 of the outgoing trunk 3| 0 extending to the trunk circuit 203TC and accessible to the outgoing selector 530. Also, the switch relay R210 completes, at the contacts 212, 213 and 215, a connection between the line conductors 032| and C322 of the incoming trunk 320 extending to the incoming selector 35!! and the line conductors C202 and 020| extending to the hybrid coil 530 in the carrier equipment 503143. Also, the switch relay R210 completes, at the contacts 213, a path, including the contacts 23|, for connecting the right-hand winding of the answer relay R260 across the line conductors 032| and C322 of the incoming trunk 322 extending to the incoming selector 3:'50, thereby to effect the seizure of the incoming selector 350, whereby it is rendered responsive to the rst digit transmitted over the line 400. Also, the switch relay R215 interrupts, at the contacts 211, a normally completed path, including the contacts 2|3, for applying battery potential by way of the impedance 228 to the 14 control conductor C3| 3 of the outgoing trunk 3 0; and completes, at the contacts 218, a substantially identical path, including the contacts 2 I3, for applying ground potential to the control conductor C3|3 of the outgoing trunk 3|0. When battery potential is thus removed from the ground -potential is thus applied to the control conductor C3|3 of the outgoing trunk 3H?, the marking of the outgoing trunk 3| 0 as idle is removed and the marking of the outgoing trunk 3|@ as busy is brought about. Accordingly, the outgoing trunk SIS, and consequently the trunk circuit 203TC, is marked as busy to the outgoing selectors 350, etc. at this time. When the incoming selector 360 is conditioned to be responsive to the first digit transmitted over the line 302 as above noted, ground potential therein is returned over the control conductor C323 of the incoming trunk 320, thereby to complete a circuit for energizing the left-hand winding of the answer relay R255 and a multiple holding circuit for energizing the upper Winding of the switch relay R2i0. The answer -relay R255 does not operate at this'timc, since it is of the shunt field type, and will not operate until the right-hand winding thereof is energized in the opposite direction over the loop circuit extending by way or the line conductors C32| and C32 2 of the incoming trunk 320 to the incoming selector 360. At this time the carrier equipment 50312 has seized the trunk circuit 203TC and the latter trunk circuit has seized the incoming selector 360, rendering it responsive to the rst digit transmitted over the line 400.

When the iirst digit is received over the line 500 the signal-in relay R555 in the carrier equipment503E follows, repeating, at the contacts 551, to the signal-in relay R230 in the trunk circuit 203TC, whereby the latter relay follows. Each time the signal-in relay R250 restores and then reoperates it interrupts and then recompletes, at the contacts 283, the previously mentioned circuit for maintaining operated the hold relay R220, thereby to cause the latter relay to remain operated during impulsing since it is of the slowto-release type. Also, each time the signal-in relay R280 restores and then reoperates it completes and then interrupts, at the contacts 282, `a circuit, including the contacts E22, for operating the dial relay R21i0, thereby to cause the latter relay to operate and remain operated during impulsing as it is of the slow-to-release type. Upon operating, the dial relay R25@ completes, at the contacts 24|, a path, including the resistor 2| 9, in multiple ywith the right-hand winding of the answer relay R255, across the line conductors C32| and C322 of the incoming trunk 325, thereby to improve impulsing to the incoming-selector 360. Finally, each time the signal-in relay R280 restores and reoperates it interrupts and then recompletes, at the contacts 28|, the previously traced loop circuit extending by way of the line conductors 032| and C322 of the incoming trunk 320, whereby the impulses of the first digit are transmitted to the incoming selector 365 in order to cause corresponding operation thereof. More particularly, the incoming selector 360 operates in response to the first digit to select the corresponding group of connectors, including the connector 310, and then operates automatically to select an idle connector in the previously selected group. For example, the incoming selector 360 may operate to seize the trunk 31| extending to the connector 3ft, whereby the connector 31B is conditioned to be responsive to the second and third digits transmitted over the line 450. At the conclusion of the first digit transmitted over the line 469 the signal-in relay R289 is retained in its operated position, thereby to retain completed the previously mentioned circuit for maintaining operated the hold relay R226 and to retain completed the previouslyY traced loop circuit extending by way of the line conductors C32| and C322 of the incoming trunk 323 to the incoming selector 366. Finally, the operated signal-in relay R286 maintains interrupted, at the contacts 282, the previously traced circuit for maintaining operated the dial relay R241), whereby the latter relay restores shortly thereafter as it is of the slow-to-release type. Upon restoring, the dial relay R249 interrupts, at the contacts 24|, the previously mentioned path, including the resistor 2|9, for short-circuiting the right-hand winding of the answer relay R265.

The second and third digits transmitted over the line 460 are received by the carrier equipment 563B! and repeated to the trunk circuit 203TC in an identical manner, whereby the second and third digits mentioned .are repeated by` the trunk circuit 263TC over the incoming trunk 320, the incoming selector 360 and the trunkV 31| to the connector 31|), in order to cause corresponding operation thereof to select the subscriber line 330 extending to the called subscriber substation T in the automatic exchange 2l), whereby the idle or busy condition thereof is tested in the connector 31D. First assuming that the subscriber substation T is idle at this time, battery potential appears upon the control conductor C334 of the subscriber line 330 causing the connector 310 to operate in order to seize the subscriber line 336, whereby ground potential in the connector 31|! is applied to the control conductor C334 of the subscriber line 333 extending to the associated line circuit 335, thereby marking the subscriber substation T as busy to the other connectors in the group, including the connector 310, having access thereto. Also, the ccnnector 313 operates to project ringing current over the line conductors C33| and C332 of the subscriber line 330 in order to operate the ringer' at the called subscriber substation T and to return ring-back tone current over the line conductors of the trunk 31| and, consequently, over the incoming selector 369, the line conductors 032| and C322 of the incoming trunk 325, and the line conductors C292 and C| to the hybrid coil 530 in the carrier equipment 593E. The ring-back tone current is passed from the hybrid coil 53B through the transmission low pass filter 54| and the input pad 542 to the modulator 543. The transmission low pass filter 54|V passes the audio frequency of the ring-back tone signal; and the input pad 542 establishes the required intensity of input of the ring-back tone signal into the modulator 543. The modulator 543 is supplied with the required intensity of 9.35 kc. carrier by the oscillator pad 541 from the carrier frequency oscillator 548 and, being of the balanced type, is now unbalanced whereby modulated 9.35 kc. carrier is passed therefrom through the output pad 544 and the attenuation equalizer 545 to the transmitting band pass lter 546. The output pad 544 controls the intensity of the modulated 9.35 kc. carrier that is supplied to the transmission line amplifier 5|1; while the attenuation equalizer 545 compensates for the disproportionate attenuation of high frequency components of the modulated 9.35 kc. carrier. Finally, the transmission band pass lter 546 passes the lower 16 side band of the 9.35 kc. carrier, blocking the upper side band thereof.

The transmission line amplifier 5|1 ampliiies the lower side band of the 9.35 kc. carrier and passes it through the transmission direction lter 5|5 and thence through the high pass line filter 5|3 to the line 490. The transmission direction filter 515 passes the carrier frequencies t0- ward the lower end of the overall carrier fre-v quency range, particularly the frequencies 6.5 to 15.95 kc.; while the high pass line lter 5|3 passes carrier frequencies over the entire carrier frequency range from 6.5 to 28.45 kc. Also, the lower pass filter 5|| blocks the passage of all carrier frequencies on the line 409 into the voice frequency equipment 5|2E; and the low pass filter 4| blocks the passage of all carrier frequencies on the line 400 into the voice frequency equipment 4|2W. The lower side band of the 6.5 kc. carrier passes from the line 406 through the high pass line filter 4| 3 and the receiving direction filter 4|4 to the receiving line amplifier 4|6. The high pass line filter 4|3 passes carrier frequenies over the entire carrier frequency range from 6.5 to 28.45 kc.; while the receiving direction lter 4|4 passes carrier frequencies toward the lower end of the overall carrier frequency range, particularly the frequencies from 6.5 to 15.95 kc. The receiving line amplifier 4|6 amplies the lower side band of the 9.35 kc. carrier and passes itV through the receiving band pass lter 42|, the attenuation equalizer 422 and the gain control 423 to the demodulator 424. The receiving band vpass filter 42| passes carrier frequencies from 6.5 to 9.1 kc.; and the attenuation equalizer 422 compensates for unequal attenuation of high frequency carrier signals on the line 400; while the gain control 423 establishes the intensity of the lower side bandv of the 9.35 kc.

carrier signal supplied to the demodulator 424.V

The demodulator 424 is supplied with 9.35 kc. carrier frequency from the carrier frequency oscillator 428 by way of the oscillator pad 421, the

oscillator pad 421 establishing the intensity of amplifier 426 and supplied to the hybrid coil 43|).l

Finally, the ring-back tone signal is returned by the hybrid coil 430 over the line conductors Y C|9I and S|92 and, consequently, over the previously traced path to the operator headset ||3 at theA switchboard |66, in order to indicate to the operator thereat that the called subscriber substation T in the automatic exchange 20 is being rung.

When the subscriber at the called subscriber substation T in the automatic exchange 20 answers the call, the connector 31D switches through to interrupt the projection of ringing current over the subscriber line 33B extending to the called subscriber substation T and the return of ringback tone current over the trunk 31| and the previously traced connection to the operator headset I |3 at the switchboard |00 in the manual exchange l0. Also, the connector 310 operates to bring aboutthe reversal of polarity over the line conductors of the trunk 31| and, consequently, over the incoming selector 360 and theff 17 line conductorsl C32| and C322 of the incoming trunk 320, whereby the right-hand winding of the answer relay R265 in the trunk circuit 203TC is poled to operate. Upon operating, the answer relay R255 interrupts, at the contacts 266, a normally completed termination circuit, including the resistor 225, the contacts 29| and 252, and the condenser 218 extending between the line conductors C252 and CZUI interconnecting the trunk circuit 2U3TC and the hybrid coil 530 in the carrier equipment 593E, in order to prepare the connection for conversation between the operator switchboard |90 in the manual exchange I and the called subscriber substation T in the automatic exchange 20. Also, the answer relay R265 completes, at the contacts 261, an obvious circuit for energizing the lower winding of the supervisory relay R285, thereby to cause the latter relay ot operate and complete, at the contacts 281, an obvious path for applying battery potential by way of the resistor 229 to the signalout conductor C253 extending to the carrier equipment 50351. This application of resistance battery potential to the signal-out conductor C203 completes an obvious circuit for operating the signal-out relay R56I; whereupon the latter relay, upon operating, completes, at the contacts 552, the plate circuit for the transmission ampliiier 563 which is supplied with 9.35 kc. carrier from the carrier frequency oscillator 548 through the oscillator pad 541. Accordingly, the transmission amplifier 553 supplies 9.35 kc. carrier by way of the signal transmission pad 554 and the signal transmission lter 555 to the transmission line amplier 5|1. The oscillator pad 541 estabf1" lishes the intensity of the 9.35 kc. carrier input to the transmission amplifier 553; and the signal transmission pad 554 establishes the inten-A sity of the 9.35 kc. carrier supplied to the transmission line amplifier 5| 1; while the signal transmission vfilter 555 insures that only the 9.35 kc. carrier is included in the input of the transmission line amplifier 5|1.

When the transmission line amplifier 5|1 receives the 9.35 kc. carrier signal it amplies the input carrier signal and passes a corresponding amplified carrier signal through the transmission direction lter 5|5 and the high pass line filter 5|3 to the line 400. The V9.35 kc. carrier on the line 45|] passes through the high pass filter 4|3 and the receiving direction filter 4|4 to the receiving line amplier 4|5. The 9.35 kc. carrier supplied to the receiving line amplifier 4&6 is amplified and passes through the signal receiving lter 45| The signal receiving filter 45| passes `the 9.35 kc. carrier and the amplified signal from the receiving amplifier 452 is rectified by the rectifier 453, in order to produce a bias that is passed to the signal bias control 454; which, in turn, governs the D. C. amplifier 455 in order to cause the D. C'. amplifier 455 to reduce the output current thereof supplied to the signal-in relay R456, whereby the latter relay restores. Upon restoring, the signal-in relay R455 completes, at the contacts 451, an obvious path for applying ground potential to the signal-in conductor C|94 extending to the trunk circuit |53TC, thereby to complete an obvious circuit for energizing the lower winding of the signal-in relay Rl therein, whereupon the latter relay operates. Upon operating, the signal-in relay Ri completes, at the contacts |82, an obvious multiple circuit for illuminating the busy lamp Ll53; and interrupts, at the contacts |81, the previously traced circuit for to .the receiving amplifier 452.

energizing the upper winding of the sleeve relay Rl in multiple with the lower winding thereof and in series with the winding of the supervisory relay R|3il in the cord circuit |5l, whereby the sleeve relay RIS@ is retained in its operated position and the supervisory relay R restores as it is of the marginal type. Upon restoring', the supervisory relay Ri3o interrupts, at the contacts |31, the previously mentioned circuit for illuminating the supervisory lamp Ll I! in the cord circuit 19|, thereby to render to the operator at the switchboard |39 answer supervision. At this 'time the operator at the switchboard |90 may converse with the subscriber at the called subscriber substation T in the automatic exchange 20 and with the subscriber at the calling subscriber substation in the manual exchange It, lsince a three-way communication connection has now been established. Then the operator may restore the talk key K593i of the cord circuit |01, disconnecting the operator headset H3 from the cord circuit 19|, whereby a connection is then completed between the calling subscriber substation in the manual exchange l5 and the called subscriber substation T in the automatic exchange 2|). At this time transmitter battery is supplied, via the upper and lower windings of the supervisory relay RMB, the back contacts of the ring key Kl and the tip and the ring of the plug Pi, to the calling subscriber substation, whereby the supervisory relay Rill@ occupies its operated position maintaining interrupted, at the contacts 4|, a circuit for illuminating the super- Visory lamp Ll |2 in the cord circuit lil Accordingly, the extinguished supervisory lamp L||2 and the extinguished supervisory lamp LI respectively indicate that the calling subscriber substation in the manual exchange Il] and the called subscriber substation T in the automatic exchange 2B are included in the established connection.

At this time audio frequency signals received from the calling subscriber substation in .the manual exchange i9 pass through the condensers |32 and |33 in the cord circuit |il| via the plug P|92 and the jack J|5| into the trunk circuit |03TC, and thence over the line conductors C|9| and C192 to the hybrid coil 435 in the carrier equipment 493W. rThe audio frequency signals then pass through the transmission low pass .filter 44| v ia the input pad 442 to the modulator 443. The transmission low pass nlter 44| insures the passage of only audio frequency signals; while the input pad 442 determines the intensity of the audio frequency signals impressed vupon the modulator 443. As previously noted, the modulator 443 is connected to the 28.45 kc. carrier oscillator 448 by the oscillator pad 441 which maintains the desired intensity of the 28.45 kc. carrier impressed upon the modulator 443. The modulator 443 is of the balanced type, whereby modulated 28.45 kc. carrier is passed through th output pad 444 and the `attenuation equalizer 445 to the transmission band pass filter 445. The output pad 444 establishes the intensity of the modulated 28.45 kc. carrier supplied to the transmission band pass filter Vr445; while the attenuation equaliser 445 compensates for the unequal attenuation of the high frequency carrier signals on the line 490. The transmission band pass filter l445 transmits the lower side band of the 23.45 kc. carrier (25.6 to 28.2 kc.) blocking the upper side band thereof and supplying the transmission line amplifier 4 1. The transmission line ampier 4|1 amplifles the lower side band of the 28.45 kc. carrier and passes V19 it through the transmission direction filter 415 and the high pass line filter 4|6 to the line 400. The lower side band of the 28.45 kc. carrier on the line 400 passes through the high pass line filter 5|3 and the receiving direction filter 5|4 to the receiving line amplier 5 I 5. The receiving line amplifier 5|5 amplifies the lower side band of the 28.45 kc. carrier and passes it through the receiving band pass filter 52|, the attenuation equalizer 522 and the gain control 523 to the demodulator 524. The receiving band pass filter 52| passes the lower side band of the 28.45 kc. carrier (25.6 to 28.2 kc); and the attenuation equalizer 522 compensates for the unequal attenuation of the high frequency carrier signals on the line 400; while the gain control 523 establishes the desired intensity of the lower side band of the 28.45 kc. carrier supplied to the demodulator 524. The demodulator 524 is connected to the 28.45 kc. carrier oscillator 528 by the oscillator pad 521, whereby the required intensity of the 28.45 kc. carrier is impressed thereupon; the demodulator 524 being of the balanced type, the envelope of the lower side band of the 28.45 kc.

carrier, determined by the audio frequency signals received from the calling subscriber substation in the manual exchange l0, is produced and passes through the receiving low pass lter 525 to the receiving amplifier 525. The receiving low pass lter 525 passes audio frequency, the audio frequency signal from the calling subscriber substation in the manual exchange I0; and this signal is amplified by the receiving amplifier 525 and supplied to the hybrid coil 530. Finally, the audio frequency signal is transmitted by the hybrid coil 530 over the line conductors C| and C202 and, consequently, over the previously traced connection, including the trunk circuit 203TC, the incoming trunk 320, the incoming selector 350, the trunk 31|, the connector 310 and the subscriber line 330 to the called subscriber substation T in the automatic exchange 20.

Also at this time audio frequency signals re-A ceived from the called subscriber substation T in the automatic exchange 20 pass over the subscriber line 330, the connector 310, the trunk 31|, the incoming selector 350 and the incoming trunk 320 to the trunk circuit 203TC, and thence by way of the line conductors C20| and C202 to the hybrid coil 530 in the carrier equipment 503E. The audio frequency signals then pass through the transmission low pass filter 54| via the input pad 542 to the modulator 543. The transmission low pass filter 54| insures the passage of only audio frequency signals; while the input pad 542 determines the intensity of the audio frequency signals impressed upon the modulator 543. As previously noted, the modulator 543 is connected to the 9.35I kc. carrier oscilllator 548 by the oscillator pad 541 which maintains the desired intensity of the 9.35 kc. carrier impressed upon the modulator 543. The modulator 543 is of the balanced type, whereby modulated 9.35 kc. carrier is passed through the output pad 544 and the attenuation equalizer 545 to the transmission band pass filter 546. The output pad 544 establishes the intensity of the modulated 9.35 kc. carrier supplied to the transmission'band pass filter 546; while the attenuation equalizer 545 compensates for the unequal attenuation of the high frequency carrier signals on the line 400. The transmission band pass filter 546 transmits the lower side band of the 9.45 kc. carrier (6.5 to 9.1 kc.) blocking the upper side band thereof and supplying the transmission 2f) line amplifier 5| 1. The transmission line amplier 5|1 amplies the lower side band of the 9.35 kc. carrier and passes it through the transmission direction filter 5|5 and the high pass line filter 5|5 to the line 400. The lower side band of the 9.35 kc. carrier on the line 400 passes through the high pass line lter 4|3 and the receiving direction lter 4|4 to the receiving line amplifier 4|5. The receiving line amplifier 4|@ amplifies the lower side band of the 9.35 kc. carrier and passes it through the receiving band pass lter 42|, the attenuation equalizer 422 and the gain control 423 to the demodulator 424. The receiving band pass filter 42| passes the lower side band of the 9.35 kc. carrier (6.5 to 9.1 kc.) and the attenuation equalizer1 422 compensates for the unequal attenuation of the high frequency carrier signals on the line 400; while the gain control 423 establishes the desired intensity of the lower side .band of the 9.35 kc. carrier supplied to the demodulator 424. The demodulator 424 is connected to the 9.35' kc. carrier oscillator 428 by the oscillator pad 421, whereby the required intensity of the 9.35 kc. carrier is impressed thereupon; the demodulator 424 being of the balanced type, the envelope of the lower side band of the 9.35 kc. carrier, determined by the audio frequency signals received from the called subscriber substation in the automatic exchange 20, is produced and passes through the receiving low pass filter 425 to the receiving amplifier 426. The receiving low pass filter 425 passes audio frequency, the audio frequency signal from the called subscriber substation in the automatic exchange 20; and this signal is amplified by the receiving amplifier 425 and supplied to the hybrid coil 430. Finally, the

'audio frequency signal is transmitted by the hybrid coil 430 over the line conductors C|9| and C|92 and, consequently, over the previously traced connection, including the trunk circuit |03TC and the cord circuit |0| to the calling subscriber substation in the" manual exchange l0.

Now assuming that the called subscriber substation T in the automatic exchange 20 is busy when the connector 310 operates in the manner previously explained to select the subscriber line 330 extending thereto, connector 310 operates in order to cause busy tone current to be returned over the previously traced connection including the trunk circuit 203TC to the hybrid coil 530 in the carrier equipment 503E. This audio frequency signal then passes through the transmitting communication channel in the carrier equipment 503E in order to cause the lower side band of the 9.35 kc. carrier to be impressed upon the line 400, in the manner previously explained, which lower side band of the 9.35 kc. carrier then passes through the receiving communication channel in the carrier equipment 403W, whereby an audio frequency signal, the busy tone signal, is returned over the trunk circuit |03TC and the cord circuit to the operator headset ||3 at the switchboard |00, in the manner previously explained. At this time the operator at the switchboard |00 advises the subscriber at the calling subscriber substation in the manual exchange I0 that the desired connection cannot be had immediately due to the busy condition of the called subscriber substation T in the automatic exchange 20, and then proceeds to vwithdraw the plugs P|03 and Pl02 of the cord circuit |0| from the associated jacks. When the plug P|02 is withdrawn from the jack J |5| the apparatus involved in theconnection extending 2i toward the called subscriber substation T in the automatic exchange 2.40 is released, in a manner more fully described hereinafter; and whenthe plug PI03 is withdrawn from the` associated jacl'; the connection to the callingsu'bscriber substation in the manual exchange I is taken down,

At the conclusion of the connection, when the subscriber at the calling subscriber substation in the manual exchange I0 replaces the receiver of the telephone instrument thereat upon its associated switchliook, the previously mentioned circuit for maintaining operated the supervisory relay RIM in the cord circuit IOI is interrupted, whereupon the latter relay restores in order to complete, at the contacts I4 I, the previously mentioned circuit for illuminating the supervisory lamp LII2 i'n the cord circuit II'II. When the supervisory lamp Ll I2 is thus illuminated it renders to the operator switchboard |00 disconnect supervision, whereupon the operator at the switchboard |00 withdraws the plug P|03 of the cord circuit Ii from the associated jack, taking down the connection to the calling subscriber substation in the manual exchange I0.

First assuming that the subscriber at the called subscriber substation T in the automatic exchange 20 replaces the receiver of the telephone instrument thereat upon its associated switchhook before the operator at the switchboard IIlIll withdraws the plug P|02 of the cord circuit I0! from the associated jack J |5| terminating the trunk circuit |03'I'C, the connector 310 operates to reverse the polarity over the trunk 31|, the incoming selector 350 and the incoming trunk 320 to the right-hand winding of the answer relay R265 in the trunk circuit 203TC, causing the latter relay to restore. Upon restoring, the answer relay R265`recompletes, at the contacts 266, the previously traced termination circuit, including the resistor 225 and the condenser 2|3, across the line' conductors C202 and C| extending into the carrier equipment 563E; and interrupts, at the contacts 261, the previously traced circuit for maintaining operated the supervisory relay R285, thereby to cause the latter relay to restore shortly thereafter, it being of the slow-to-release type. Upon restoring, the supervisory relay R285 interrupts, at the contacts 281, the previously traced path for applying battery potential by way of the resistor 229 to the signal-out conductor C203 extending to the carrier equipment 503E in order to bring about the restoration of the signalout relay R56| therein. Upon restoring, the signal-out relay R56I interrupts, at the contacts 562, the plate circuit for the transmitting amplifier 563 in order to interrupt the transmission, over the transmitting signal channel in the carrier equipment 503E, of the 9.35 kc. carrier to the line 460. When the 9.35 kc. carrier is thus removed from the line 400 and, consequently, from the receiving signal channel in the carrier equipment 403W, the signal-in relay R456 therein reoperates, in the manner previously explained, interrupting, at the contacts 451, the path for applying ground potential to the signal-in conductor CISA extending to the trunk circuit |03TC. When ground potential is thus removed from the signal-in conductor C|94 the previously traced circuit for energizing the lower winding of the signal-in relay RI 80 in the trunk circuit I 03TC is interrupted, causing the latter relay to restore. Upon restoring, the signal-in relay RI 80 interrupts, at the contacts |82, the previously mentioned multiple circuit for illuminating the busy lamp LI53; and recompletes, at the contacts ISI,

22 the previously traced circuit for energizing the upper winding of the sleeve relay R| in multiple with the lower winding thereof and, consequently, in series with the winding ofthe supervisory relay RIB!! in the 4cord circuit IOI. When this 10W resistance circuit is thus completed the supervisory relay RI30 operates to recomplete, at the contacts IBI, the previously mentioned circuit for illuminating the supervisory lamp LI I I, thereby to render to the operator at the switchboard |00 disconnect supervision.

Upon receiving disconnect supervision the operator at the switchboard |00 withdraws the plug P|02 of the cord circuit |0I from the associated jack J I5I, thereby interrupting the series circuit for maintaining operated the supervisory relay R|30 in the cord circuit |0I and the sleeve relay RISE) in the trunk circuit I03TC. At this time the supervisory relay RI36 restores, interrupting, at the contacts I3 I, the previously mentioned circuit for illuminating the supervisory lamp LII I, whereby the cord circuit IOI is restored to its. normal condition. Also, when the plug P|02 is withdrawn from the jack J I5I, the contacts I5.I' thereof are operated into open circuit relation, interrupting the previously mentioned circuit for maintaining operated the jack relay RI10, in order to cause the latter relay` to restore shortly thereafter, it being of the slow-to-release type. Upon restoring, the sleeve relay RI60 interrupts, at the contacts I6I, the previouslyl traced path for applying battery potential by way of the resistor |61 to the signal-out conductor CI93 extending to the carrier equipment 403W; and recompletes, at the contacts |64, the previously mentioned termination circuit, including the resistor I 66, across the line conductors CIS! and Cl92, extending to the carrier equipment 403W. Also, the jack relay Ril interrupts, at the contacts |13, the previously mentioned original circuit for illuminating the busy lamp LI 53, thereby to indicate that the trunk circuiti 03TC is idle and available for use at this time.

When resistance battery potential in the trunk r circuit I 03'I'C is removed from the signal-out conductor CI93 the previously traced circuit for maintaining operated the signal-out relay R46 I in the carrier equipment 463W is interrupted. Upon restoring, the signal-out relay R46I interrupts, at the contact-s 452, the plate circuit for the transmitting ampliiier 463 in order to interrupt the transmission, over thetrans-mitting signal channel in the carrier equip-ment 403W, of the 28.45 kc. carrier to the line l400. When the 28.45 kc. carrier is thus removed from the line 400 and, consequently, from the receiving signal channel in the carrier equipment .563E, the signal-in relay R556 therein reoperates, in the manner previously explained, interrupting, at the contacts 551, the path for applying ground potential to the signal-in conductor C204 extending to the trunk circuit 2:33TC. When ground potential is thus removed from the lsignal-in conductor 020e the previously traced circuit for maintaining operated the signal-in relay R280 in the trunk circuit 2235TC is interrupted, causing the latter relay to restore.

Upon restoring, the signal-in relay R280 interrupts, at the contacts 28|, the previously traced A loop circuit extending by way of the incoming trunk 320 to the incoming selector 360 and thence by way of the trunk 31| to the connector 316, thereby to bring about the release of the connector 310i. When the connector 310 is thus released ground potential therein is removed from the control conductor C332 of the 'subscriber line 330 extending to the line circuit 335, whereby battery potential in theline circuit 335 is applied to the control `conductor C334, thereby marking the subscriber line 030 extending to the subscriber substation T as idle to the connectors in the group, including the connector 310, having 4access thereto. Also, when the connector 310 is released ground potential is removed from the control conductor of the trunk 31|, bringing about the release oi' the'incoming selector 300 and the consequent removal of ground potential from the control conductor C323 of the incoming trunk 320. When ground potential is 'removed from the control conductor C323 of the incoming trunk 320 the previously traced multiple circuits for energizing the left-hand winding of the answer relay R265 and the upper winding of the switch relay R210 are interrupted. Also, upon restoring, the signal-in relay R280 recompletes, at the contacts 282, the previously traced circuit for operating the dial relay R200; and interrupts, at the contacts 283, the previously mentioned circuit for maintaining operated the hold relay R220, whereby the latter relay restores shortly thereafter it being of the slow-to-release type.

Upon restoring, the hold relay R220 interrupts, at the contacts 222, the previously traced circuit for maintaining operated the dial relay R240, thereby to cause the latter relay to restore shortly thereafter, it being of the slow-to-release type. Further, the hold relay R220 interrupts, at the contacts 224, the previously traced circuit for energizing the lower winding of the switch relay R210, thereby to cause the latter relay to restore. Upon restoring, the switch relay R210 interrupts, at the contacts 212, '213 and 215, further points in the previously traced connection between the line conductors C202 and C20! extending to the carrier equipment 503E and the line conductors 032| and C322 of the incomingr trunk 320 extending to the incoming selector 300; `and recompletes, at the contacts 21| and 210, the previously traced normal connections, including the contacts 291 and 290, between the line conductors C202 and C201 and the line conductors 03| and C3|2 of the outgoing trunk 3|0 accessible to the outgoing selector 350. Further, the switch relay R210 interrupts, at the contacts 218, the previously traced path for applying ground potential to the control conductor CSES of the outgoing trunk 3|0; and recompletes, at the contacts 211, the previously traced normal path for applying battery potential by way oi the impedance 228 to the control conductor C3|3, thereby to mark the outgoing trunk 3|() as idle to the outgoing selectors 356, etc. having access thereto. At this time the trunk circuit 203TC is completely released and available for further use.

Now assuming that the operator at the switchboard withdraws the plug P|02 oi the cord circuit i0! from the associated jack J |5| terminating ythe trunk circuit I03TC before the subscriber at the called subscriber substation 'i' in the automatic exchange 20 replaces the receiver 24 tor C |93 in order to bring about the restoration of the signal-out relay R06! in the carrier equipment MSW, in the manner previously noted. Upon restoring, the signal-out relay Rill interrupts, at the contacts IESE, the plate circuit for the transmitting amplifier 003 in order to interrupt the transmission over the transmitting signal channel in the carrier equipment 403W, of the 28.45 kc. carrier to the line 000. When the 28.45 kc. carrier is thus removed from the line 400 and, consequently, from the .receiving signal channel in the carrier equipment 503E, the signal-in relay R55|i therein reoperates, in the manner previously' explained, interrupting, at the contacts 551, the path for applying ground potential to the signal-n conductor C202 extending to the trunk circuit 203TC. When ground potential is thus removed from the signal-in conductor C205. the previously traced circuit for o the telephone instrument thereat upon its maintaining operated the signal-in relay R280 in the trunk circuit 203TC is interrupted, causing the latter relay to restore.

YUpon restoring, the signal-in relay R280 completes, at the contacts 282, a circuit, including the contacts 21B and 286, for energizing the upper winding or" the supervisory relay R285; and interrupts, at the contacts 28 i, the previously traced loop circuit extending between the right-hand winding of the answer relay R255 and the connector 310 in order to bring about the release of the connector 310 and the incoming selector 360, in the manner previously explained. When the connector 310 is thus released in the event the subscriber at the called subscriber substation T has not replaced the receiver of the telephone instrument upon its associated switohhook at this time, the line circuit 335 individual to the subscriber line 330 operates in order to lock the subscriberoline 330 out of service and to mark it as busy to the connectors in the group, including the connector 320, having access thereto.

When the incoming selector 300 is thus released ground potential is removed from the control conductor C323 of the incoming trunk 320, thereby to interrupt the previously traced multiple circuits for energizing the left-hand winding of the answer relay R265 and the upper winding of the switch relay R210. At this time the answer relay R2 E5 restores, interrupting, at the contacts 261, the previously mentioned circuit for energizing the lower Winding of the supervisory relay R285; however, the latter relay is maintained in its operated position at this time, due to the completed circuit forenergiaing the upper Winding thereof. Further, upon restoring, the signalin relay R280 interrupts, at the contacts 283, the previously mentioned circuit for maintaining operated the hold relay R220, thereby to cause the latter relay to restore shortly thereafter, it being of the slow-to-release type. Finally, the signalin relay R280, upon restoring, completes, at the contacts 282, the previouslyV traced circuit for operating the dial relay R240. Upon restoring, the hold relay R220 brings about the restoration of the dial relay R240 shortly thereafter, as previously noted, and interrupts, at the contacts 224, the previously traced circuit for energizing the lower winding of the switch relay R210 in order to cause the latter relay to restore. Upon restoring, the switch relay R210 interrupts, at the contacts 210, the previously traced circuit for energizing the upper winding of the supervisory relay R285, thereby to cause the latter relay to restore shortly thereafter, it being of the slowto-release type. Upon restoring, the supervisory relay R235 interrupts, at the contacts 287, the previously traced path for applying battery potential by way of the resistor 229 to the signalout conductor C233 extending to the carrier equipment 533B?. At this time the trunk circuit 23TC is completely released and is available for further used.

When ground potential is removed from the control conductor C253 the previously traced circuit for maintaining operated the signal-out relay Rl in the carrier equipment 53351 is interrupted, causing the latter relay to restore. Upon restoring, the signal-out relay R5i interrupts, at the contacts 532, the plate circuit .for the transmitting amplier 563 in order to interrupt the transmission, over the transmitting signal channel in the carrier equipment 53313, of the 9.35 kc. carrier to the line 433. When the 9.35 kc. carrier is thus removed from the line 405 and, consequently, from the receiving signal channel in the carrier equipment 403W, the signal-in relay R456 therein reoperates, in the manner previously explained, interrupting, at the contacts Mil, the path for applying ground potential to the signal-in conductor C|94 extending to the trunk circuit IGSTC. When ground potential is thus removed from the signal-in conductor C|9l the previously traced circuit for energizing the lower winding of the signal-in relay Ri in the trunk circuit |33TC is interrupted, causing the latter relay to restore. Upon restoring, the signal-in relay Rlil interrupts, at the contact' |62, the previously mentioned alternative cir` cuit for illuminating the busy lamp LI 53, in order to extinguish the busy lamp U53, thereby tf indicate that the trunk circuit |3TC is idle an available i or further use at this time.

Considering now the extension of a connection between the automatic exchange 2|] and the manual exchange i3, and assuming that a call is initiated at the calling subscriber substation T in the automatic exchange 2|), the line circuit 335 individual to the subscriber line 333 extending to the calling subscriber substation T operates in order to place battery potential upon the control ccnductor C333 of the subscriber line 333 and ground potential upon the start conductor C335 extending to the distributor 33o associated with the group of finder-selector links, including the link illustrated, and comprising the finder 352 and the selector 34|. The distributor 335 assigns an idle one of the finder-selector links mentioned, such, for example, as that illustrated, and initiates operation of the inder 332 thereof, whereby the finder 332 operates to seize the subscriber line 330 extending to the calling subscriber substation T bringing about further operation of the line circuit 335, whereupon ground potential is placed upon the control conductor C333 of the subscriber line 335 in order to mark the subscriber line 333 as busy to the connectors in the group, including the connector 313, having access thereto. Also, the line circuit 335 releases the distributor 345 and the selector 35| is conditioned to be responsive to the first digit dialed at the calling subscriber substation T. Furthermore, it is noted that the calling subscriber substation T is included in a ten line group rendered a particular class of service, whereby the set of switch springs S33@ in the nder 332 is actuated into engagement incident to operation of the nnder to seize the subscriber line 339 extending to the calling subscriber substation T. When the set or" switch springs S35 is thus actuated the class tone conductor Cll, to which a class tone indicative of the class of the calling subscriber substation T is subsequently impressed, is connected by way of the condenser 345 to the set of switch springs S343 in the selector 34|; which set of switch springs S343 is operated further to connect the tone conductor C345 to the control conductor of a seized trunk, only in the event the selector 34| is operated inaccordance with a predetermined nrst digit indicative of a trunk call. Thus, it will be understood that when the selector 33| is operated to seize the trunk 3l! extending to the connector 310 when a local call is extended by way of the lnder-selector link illustrated that the set of switch springs S333 is not operated toA connect the class tone conductor CME to the control conductor of the trunk 37| extending to the connector 310, since such operation would be of no utility in conjunction with the extension of a local call from the calling subscriber substation T.

However, continuing with the extension oi the present call from the calling subscriber substation T when the subscriber thereat dials the first digit into the selector 33|, the selector 34| operates in order to select rst the corresponding group of trunks and then an idle trunk therein. For example, the selector 35| may operate to select the` particular trunk 35i extending to the outgoing selector 353 in response to the rst digit dialed therein, whereby the loop circuit from the calling subscriber substation T is extended into the outgoing selector 355. Also, since this is a trunk call the set of switch springs S333 in the selector 34| is actuated into engagement, thereby to extend the class tone conductor C335 over the control conductor of the trunk 35| to the control conductor of the outgoing selector 355. rlhe outgoing selector 350 then responds to the second digit dialed at the calling subscriber substation T, selecting rst the corresponding group of outgoing trunks and then an idle outgoing trunk therein. For example, the outgoing selector 353 may select the outgoing trunk 3|0 extending to the trunk circuit 233TC, whereby there is completed a loop circuit extending from the calling subscriber substation T to the line relay R290 in the trunk circuit 233TC; and the control conductor of the outgoing selector 355 is connected to the control conductor C3i3 of the outgoing trunk 3m, for a purpose more fully explained hereinafter. In passing, it is noted that the outgoing trunk 3i@ is marked as idle to the outgoing selector 353 by the application of battery potential thereon over the previously traced path, including the contacts 2i3 and 2li and the impedance 228.

The loop circuit mentioned extends from ground by way of the lower winding of the line relay R233 and the contacts 239 to the line conductor C3l2 of the outgoing trunk 3|ll, and from battery by way of the upper winding of the line relay R253@ and the contacts 291 to the line conductor G3i of the outgoing trunk 3 E; and thence by Way or" the outgoing selector 350, the trunk 35|, the selector 33|, the finder 342 and the subscriber line 335 to the calling subscriber substation T. When this loop circuit is thus completed the line relay R233 operates to complete, at the contacts 232, a path for applying battery potential by way of the resistor 229 to the signal-out con ductor C2 33 extending into the carrier equipment 533B?, in order to bring about operation of the signal-out relay R55! therein. Also, the line relay R233 interrupts, at the contacts 29|, the previously traced termination circuit, including 

